Why tractors have bigger tires at the rear axles?





Tractor rear wheels are wide because of multiples reasons
  • Tractors is basically an off-road vehicle which don't have to carry passenger on road.
  • Tractor run in farms in soft soil and mud also so it must have better traction between tyre and road surface.
  • Tractor rear tyres are used to pull heavy load not only to push tractor farward.
  • While doing agriculture work tractor require lot of traction force which is provided by wide surface of wheel and grips on wheel surface.
  • Wide tyres also maintain the Center Of Gravity of the tractor.
  • Tractors are not designed to run fastly instead of it is design to run slowly but to provide high torque.
And now, it's not true that all tractors are provided with big rear tyres. Only 2WD tractors are provided with big rear wheels (in 4WD all wheels are of same size ) because of following reasons.
  1. As in 2WD tractor only rear wheels are available for traction purpose and we know  F = AC + W tan($)
    where W = weight on wheel
               A = contact area of wheel
               F = tractive force
    So to increase tractive force i.e. F we should increase weight i.e. W on wheels and to sustain that much load tyre diameter should be big enough.
  2. Tractor center of gravity (C.G.) lies towards rear axle (approx. 1/3 of wheelbase from rear axle) and so to sustain that much load tyre size should be bigger.
  3. Also from above equation as contact area (A) increased, tractive force (F) also increased, that is also a reason by which rear wheels are bigger in size.


    And I want to mention one more thing that front tyres in 2WD are smaller because they are towed wheels and used only for steering purpose and small wheels require less effort in steering.

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